Wireless local area networks (WLAN) are generally known in the art. For example, one embodiment of a WLAN is described in the IEEE 802.11 standard, entitled “Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications, 1999 Edition.” The IEEE 802.11 standard specifies both the PHY layer and the MAC sub-layer of the Open Systems Interconnection (OSI) Reference Model of the International Standards Organization (ISO).
The MAC protocol specified by the IEEE 802.11 standard can operate in a distributed coordination function (DCF) mode. The DCF mode governs access to the communications medium through a carrier sense multiple access with collision avoidance (CSMA/CA) scheme. FIG. 1 illustrates the DCF mode's CSMA/CA scheme. According to the CSMA/CA scheme, a station generally cannot transmit a frame until it senses that the channel has been idle for at least a DCF inter frame space (DIFS) 110. Because more than one station may be ready to transmit a packet once a DIFS has transpired, the DCF mode employs a collision avoidance mechanism. The collision avoidance mechanism requires all stations to generate a random backoff period 120 (Backoff Time) for an additional deferral time before transmitting a packet. The station that generates the shortest Backoff Time transmits a packet when the Backoff Time has transpired 130. The other stations in the WLAN cease to contend for the communications medium once they detect that the communications medium is busy 140.
The CSMA/CA scheme used by the DCF mode is a best-effort service because it does not provide a mechanism to ensure quality of service (QoS) for time-bounded packet transmissions. Multimedia content that is transmitted by a best-effort service packet transmission method may be subject to unpredictable delays and jitters. These delays and jitters can be greatly reduced if multimedia packets are given priority by the packet transmission method.
IEEE 802.11 Task Group E has been working to define an enhanced DCF mode. Task Group E has primarily focused on two methods for enhancing the DCF mode. The first method is to increase the length of the average Backoff Time for stations seeking to transmit low priority packets. The second method is to require stations with low priority packets to wait longer than a DIFS before generating a Backoff Time. Each of these methods increases the average time a station with a low priority packet must wait before transmitting the packet.
While these proposals privilege high priority packets in accessing the communications medium, they also raise new concerns. First, due to the random nature of the Backoff Time, neither of these methods guarantee that a high priority packet will always be transmitted before a low priority packet. Also, since Backoff Times are, on average, longer for low priority packets than under the standard DCF mode, the network as a whole exhibits longer delays for typical packet transmission.
The European Telecommunications Standards Institute (ETSI) has specified a standard for WLANs that is similar in scope to the IEEE 802.11 standard. The ETSI standard is named the High Performance Radio Local Area Network protocol (HIPERLAN/1). Like the IEEE 802.11 standard, HIPERLAN/1 specifies a slotted system, that is, transmission attempts take place at discrete instances in time. A slot is the basic time unit in a slotted system. HIPERLAN/1 provides for the prioritization of the transmission of packets through an Elimination Yield Non-pre-emptive Priority Multiple Access (EY-NPMA) mechanism.
FIG. 2 illustrates that the EY-NPMA mechanism comprises three phases: prioritization 210, contention 220, and transmission 230. The prioritization phase lasts from one to five slot intervals 210. Each slot interval of the prioritization phase corresponds with one of the five priority levels defined by HIPERLAN/1. The first slot interval of the prioritization phase 240 corresponds with the highest priority a packet can have and the fifth slot level 250 corresponds with the lowest priority a packet can have. A station that is ready to send a packet will transmit a signal one slot interval in duration, during the slot interval that corresponds with the priority level of the packet 250. When a station transmits during one of the slot intervals of the prioritization phase, all stations not concurrently transmitting cease to contend for transmission. Because the slot intervals are arranged in decreasing order of priority, the first station to transmit is necessarily ready to transmit a packet of higher priority than those stations that have not yet transmitted. Thus, EY-NPMA ensures that higher priority packets are transmitted before lower priority packets. The length of the EY-NPMA prioritization phase, however, can vary with each transmission cycle.